Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system, a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions. For example, a turbocharged engine system may include a low-pressure (LP) EGR system which recirculates exhaust gas from the exhaust system to the intake passage upstream of a turbocharger compressor. Accordingly, exhaust gas may be recirculated into a low-pressure air induction system upstream of the compressor, resulting in a compressed mixture of fresh intake air and EGR downstream of the compressor. An EGR valve may be controlled to achieve a desired intake air dilution, the desired intake air dilution based on engine operating conditions.
However, due to the small differential pressures inherent to LP-EGR loops, turbocharged engine systems may also include a LP intake throttle to increase the differential pressure such that higher EGR rates can be achieved. There are competing requirements that constrain the degree of throttling. On one hand, excessive throttling unnecessarily increases fuel consumption. On the other hand, too little throttling can cause the system to operate at particularly low differential pressures, which necessitates high control gains and thereby reduces the control system robustness to disturbances.
The inventors herein have recognized the above issue and have devised various approaches to address it. In particular, systems and methods for controlling an LP intake throttle and an LP-EGR valve are disclosed. In one example, a turbocharged engine method, comprises: responsive to a differential between intake and exhaust pressure below a threshold, adjusting a LP-EGR valve while adjusting a LP intake throttle to regulate a LP-EGR flow rate and the differential to respective setpoints; and responsive to the differential above the threshold, saturating the LP-EGR valve to minimize the differential while actuating the throttle to regulate the flow rate to its setpoint. In this way, control of the LP-EGR system may be more robust, require less actuator movement, and increase fuel economy.
In another example, a turbocharged engine method, comprises: responsive to a differential between intake and exhaust pressure below a threshold, adjusting a LP-EGR valve while adjusting a LP intake throttle to regulate a LP-EGR flow rate and the differential respectively to a flow setpoint and a differential setpoint; and responsive to the differential above the threshold, in a first mode saturating the LP-EGR valve to minimize the differential while actuating the throttle to regulate the flow rate to the flow setpoint, and in a second mode, saturating the intake throttle to minimize the differential while actuating the LP-EGR valve to regulate the flow rate to the flow setpoint. In this way, the control system may be more robust to disturbances at very low differential pressures and fuel consumption due to excessive throttling may be decreased.
In another example, an internal combustion engine system comprises: a turbocharger including a compressor connected to a turbine, the compressor in communication with an intake manifold of the engine and the turbine in communication with an exhaust manifold of the engine; a low-pressure (LP) exhaust gas recirculation (EGR) passage including an EGR valve and an intake throttle connecting the intake manifold and the exhaust manifold, said EGR valve responsive to an EGR valve control signal and said intake throttle responsive to an intake throttle control signal for regulating a flow rate into said intake manifold and a differential pressure in said LP-EGR passage; a controller configured with instructions stored in non-transitory memory that when executed, cause the controller to: generate a flow rate error based upon a reference flow rate and a measured flow rate; generate a differential pressure error based upon a reference differential pressure and a measured differential pressure; calculate a minimum and a maximum achievable flow rates; apply the minimum and the maximum achievable flow rates as anti-windup limits to a first proportional-integral controller; execute the first proportional-integral controller to generate an adjusted flow rate setpoint responsive to the flow rate error; calculate a minimum and a maximum achievable differential pressures responsive to the adjusted flow rate setpoint; apply the minimum and the maximum achievable differential pressures as anti-windup limits to a second proportional-integral controller; execute the second proportional-integral controller to generate an adjusted differential pressure setpoint responsive to the differential pressure error; execute a linearization controller to generate an EGR valve actuator position and a LP intake throttle actuator position responsive to the adjusted flow rate setpoint and the adjusted differential pressure setpoint; and actuate the EGR valve to the EGR valve actuator position and the LP intake throttle to the LP intake throttle actuator position. In this way, control of the LP-EGR valve and the LP intake throttle can automatically switch between a multivariable control mode that improves robustness at very low differential pressures and a chained-actuator control mode that minimizes fuel consumption.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.